1. Field of Invention
The present invention relates to methods and processes for fabricating photoelectric devices, and more particularly to methods and processes for fabricating the back face substrate for silicon wafer solar cells.
2. Description of the Related Art
In most current embodiments of solar cell technology, a solar cell comprises a silicon wafer built on a substrate (the “back face substrate”). In this respect, a silicon wafer solar cell closely resembles other silicon wafer semiconductor devices. Accordingly, certain methods and technologies used to fabricate silicon wafers for use in conventional semiconductor devices are also used to fabricate silicon wafers for solar cells. However, the choice of materials for the back face substrate of a solar cell hinges upon different criteria than the choice of substrate material for the back face substrate of a conventional semiconductor device. For example, when fabricating a silicon wafer with a back face substrate for a conventional semiconductor device, as for example for use in an integrated circuit, it is usually desirable for the substrate material to be an electrical insulator. By contrast, with a solar cell, it is desirable for the back face substrate to be an electrical conductor. Thus, the back face substrate of a solar cell is often referred to as a “back face conductor”.
In order to improve silicon utilization and reduce the material costs of producing silicon wafer solar cells, the trend in the industry has been to reduce the thickness of silicon wafers as much as is practical. Wafer thickness of between 180 microns and 200 microns is typical of the present state of the technology. Customary silicon wafer specifications at present call for a square-faced wafer that is slightly over 12 millimeters on each side of the square; thus, the length of the wafer, on any side or diagonal, is considerably greater than the thickness of the wafer.
Silicon wafer solar cells are fabricated using processes that require the silicon and the substrate material or materials to attain high temperatures—usually several hundred degrees Celsius, with the exact temperatures varying depending on the type of fabrication process used and the nature of the substrate material, among other considerations. When a silicon wafer solar cell cools after fabrication, the solar cell often will experience bowing due to a difference in the coefficient of thermal expansion (CTE) of the substrate material versus the CTE of the silicon. As the substrate material cools and contracts to a greater degree than the silicon, the contracting substrate material pulls the silicon into a curved or bowed shape. The greater the difference between the CTE of the substrate material and the CTE of silicon, the greater the bowing. Additionally, bowing generally will be greater with thinner wafers, as thinner wafers generally flex more easily than thicker wafers.
Significant bowing can damage a silicon wafer, for example by leading to separation between the silicon layer of the solar cell and the back face substrate, or by causing the silicon wafer to crack. Therefore, it is desirable to limit bowing as much as possible while still producing a thin silicon wafer with a conductive back face substrate. In addition to bowing during the fabrication process, a silicon wafer used as a solar cell may also experience bowing due to changes in temperature during use. Therefore, a silicon wafer solar cell should be designed to minimize bowing due to extremes of summer and winter weather. In particular, a silicon wafer solar cell should be designed to withstand exceptionally low winter temperatures, which may be infrequent but can nevertheless cause failures such as those discussed above when they do occur.
One approach to limiting the degree of bowing in a finished silicon wafer solar cell is to select materials for fabrication of the solar cell which decrease as much as is feasible the mismatch between the CTE of the silicon layer and the CTE of the back face substrate. With conventional semiconductor devices, practitioners have used for the back face substrate various ceramic materials that have a CTE close to the CTE of silicon. However, this class of substrate materials is generally ill-suited for use in a solar cell because the ceramic materials with a low CTE close to the CTE of silicon are generally insulators and are poor electrical conductors. As discussed above, with a solar cell, it is desirable for the back face substrate to be an electrical conductor.
As electrical conductors, the metals shown in the Table 1 as follows make attractive candidates for use as conductive back face substrates:
TABLE 1MATERIALS, COEFFICIENTS OF THERMALEXPANSION, AND MELTING POINTSCTEMelting PointMaterial(PPM per ° C.)(° C.)Silicon (Si)31414Platinum (Pt)91770Gold (Au)141063Copper (Cu)171084Silver (Ag)18961Aluminum (Al)23660
However, essentially all of the candidate metals have a high mismatch of CTE as compared to silicon. Gold and platinum have high melting points and would present high material costs for use in silicon wafers. Copper likewise has a high melting point. Silver has a slightly lower melting point, but silver's lack of elongation and relatively high cost also make it impractical to use as a substrate material for a conducting back face substrate. Aluminum is attractive as a material for a conductive back face, as it has a low melting point and is a good electrical conductor. However, Aluminum by itself as a substrate presents the possibility of significant bowing because of the great difference in CTE between silicon (which is approximately 3 parts per million per degree Celsius) and aluminum (which is approximately 23 parts per million per degree Celsius).
In light of the above, it is desirable for the back face conductor of a solar cell to be highly conductive electrically. It is also desirable for the back face conductor to be highly reflective in the ultraviolet (UV) to infrared (IR) range of the electromagnetic spectrum. A back face conductor is desired which is able to survive and operate in a wide range of thermal conditions, including cold winter conditions and summer heat. Additionally, for solar cells to be competitive with technological alternatives, it is desirable to produce a solar cell using materials for the back face conductor which are not prohibitively expensive.